At present, liquid crystal display elements are now widely used due to their excellent characteristics such as low voltage actuation, thin model display, or the like. Examples of the display systems of conventional liquid crystal display elements, particularly small-to-medium-sized elements, include TN (twisted nematic), STN (super twisted nematic), active matrix based on TN (TFT: thin film transistor), and the like, and they utilize liquid crystal compositions having positive dielectric anisotropy values.
However, these display systems have a narrow viewing angle as one of their defects, and an improvement thereof has become a major issue in accordance with the currently increasing demand for larger liquid crystal panels. As a solution to this, display systems such as vertical alignment mode, IPS (In Plane Switching), and the like have recently been newly put to practical use. The vertical alignment mode is a system to improve the viewing angle by utilizing vertical alignment of liquid crystal molecules, in which a liquid crystal composition having a negative dielectric anisotropy value is used. IPS is a system to improve the viewing angle by switching liquid crystal molecules using a horizontal electric field in parallel with glass substrates, in which a liquid crystal composition having a positive or negative dielectric anisotropy value is used. Thus, the vertical alignment mode and IPS, which are display systems effective for improving the viewing angle, require liquid crystal compounds and liquid crystal compositions having negative dielectric anisotropy values, which have become strongly demanded. However, liquid crystal compounds or liquid crystal compositions having negative dielectric anisotropy values have not been sufficiently developed, because conventional display systems mainly use liquid crystal compositions having positive dielectric anisotropy values as described above.
As a liquid crystal compound having a negative dielectric anisotropy value, a liquid crystal compound having a 2,3-difluorophenylene skeleton (see Patent Document 1) and a liquid crystal compound having a 3,4-difluoro-5,6,7,8-tetrahydronaphthalene skeleton (see Patent Document 2) are disclosed. However, the absolute values of the dielectric anisotropy values of these compounds are not necessarily sufficiently large, which prevents the progress of the development of liquid crystal display elements, so a compound having a large absolute value of negative dielectric anisotropy value is required to be developed.
As a compound having a large absolute value of dielectric anisotropy, a liquid crystal compound having a 1,7,8-trifluoronaphthalene skeleton is disclosed (see Patent Document 3). However, although this compound has a large absolute value of the dielectric anisotropy, the refractive index anisotropy thereof is large because the compound has a naphthalene skeleton. Therefore, it is difficult to apply the compound to a liquid crystal composition having a small refractive index anisotropy.
On the other hand, application of a compound having a chroman ring to a liquid crystal material has already been known (see Patent Documents 4 and 5). However, the compounds described in these documents are intended to be used for ferroelectric liquid crystal compositions, because the compounds have an unsubstituted chromam ring as well as an optically-active group. Moreover, there is no disclosure with respect to negative dielectric anisotropy in these documents. Also, chroman-based compounds which are intended for conventional nematic liquid crystal compositions and in which an optically-active group is not essential are known (see Patent Document 6). However, the described compounds are compounds each having an unsubstituted chroman ring, and seem to be intended for ferroelectric liquid crystal compositions. Moreover, most of the compounds have positive dielectric anisotropy, and compounds having negative dielectric anisotropy are not disclosed.
As mentioned above, a liquid crystal material having a large absolute value of negative dielectric anisotropy and small refractive index anisotropy has been required to be developed.
(Patent Document 1) German Laid-Open Patent Application No. 3906019
(Patent Document 2) German Laid-Open Patent Application No. 19522145
(Patent Document 3) German Laid-Open Patent Application No. 19522195
(Patent Document 4) Japanese Laid-Open Patent Application No. Hei 5-25158
(Patent Document 5) Japanese Laid-Open Patent Application No. Hei 6-256337
(Patent Document 6) Japanese Laid-Open Patent Application No. Hei 6-256339